Random ideas follow:
Humanity is easily the dominant species in the galaxy, having come to sentience earlier than other species and thus had much more time to expand in power and population. The Mon Calamari are a distant second. Their entry on the side of the rebellion after the destruction of the Death Star-class Mobile Battlestation is a major boost to The Alliance to Restore the Republic.
The Sith are an order of evil Darkside Force users, former Jedi. They flee persecution into the Unknown Regions, where their ability to control the desires and ethics (but not specific actions) of others allows them to build up a great empire amongst the many non-human peoples of the area. Battle Meditation is a Sith invention and specialty. They are much less choosy in training others in Force techniques and powers. As a result the Armed Forces of the Sith, despite their vast numerical inferiority compared to those of The Republic, contains many more Force Users. As former Jedi, they wield the “lightsaber” or “laser sword”; A close combat energy melee weapon.
The Sith Wars:
Basically the same as Dark Horse Comics Tales of the Jedi, Dark Lords of the Sith, and the video games KOTOR and KOTOR2. With the difference being the above mentioned large numbers of Force trained warriors on the side of the Sith.
Darth Vader, a Dark Jedi in the service of The Galactic Empire, claims the mantle Dark Lord of the Sith millennia later, after rediscovering some of the orders techniques. He plans to restore the order, believing only a loyal order of Force users can restore order (no pun intended) to the galaxy, but fears The Emperor’s wrath should he put those plans in motion. The Emperor prefers a virtual monopoly on Force knowledge and techniques and, although he has learned many Sith techniques himself, does not consider himself the heir to their order.
The Mandalorians
They worship two things: war and technology
Powered Armour and semi-sentient Basilisk War Droids are features unique to their civilization.
Decimated after The Second Sith War, the Mandalorian Exodus occurs. Mandalorians disappear from Known Space until The Clone Wars